This does not mean that your dog is covered in fleas it means he is sensitive to the flea saliva & it would take only 1 or 2 fleas to bite to start the allergic reaction. After the first bite your dog may still show reaction for up to 2-3 weeks.
treat the environment & all other animals in the house also.
Sometimes with fleas & flea allergy dogs A 2 pronged approach is necessary.
I have a range of treatment programs for allergic skin problems in dogs. Your dog would be best treated with the Acute Allergy Mix.
It is an internal treatment which reduces the reactivity and will make him more comfortable without the sides effects of the chemical treatments. My mix also works toward reducing the underlying sensitivity so he can cope better and better if the circumstances return.
There is also a Skin Irritant Cream which is useful both to heal the damage done to the skin from the irritation and scratching and the allergic response as well. Or you can simply make up a Chamomile Tea place in fridge to cool and spray or dab on irritated areas to calm the skin.
If your dog's problems are ongoing it's likely there are blood toxicity problems involved but these type of skin problems are treated with blood cleansers.